The invention relates to a method of transmitting a measurement signal between a measuring unit and a control unit.
In automation technology and process control technology, measured variables are frequently recorded on various process units and passed to a more remote control unit. The control unit controls the process cycle on the basis of the information about the state of the process units. The measured variables are recorded on the process units by means of measuring units and are converted into measurement signals which are transmitted to the central control unit. The measurement signals can assume continuously varying levels or else discrete levels, depending on whether a continuous measured variable or a discrete measured variable is recorded.
As an example, a limit switch for the filling level in a tank container can assume only two switching states, xe2x80x9climit level reachedxe2x80x9d and xe2x80x9climit level not yet reachedxe2x80x9d. This information is transmitted to the control unit as a discrete measurement signal which is uniform over time. If the switching state of the limit switch changes, e.g. because the liquid reaches the limit level, then the measurement signal changes accordingly.
If a malfunction occurs in the measuring unit, during which the measuring unit continues to output the original uniform measurement signal but no longer reacts to changes in the measured variable, then the control unit is not able to recognize the malfunction. The control unit continues to assume that the transmitted measurement signal is valid and, on the base of this misinformation, controls the process cycle incorrectly. Process control based on an invalid measurement signal results in unwanted erroneous process reactions. This needs to be prevented, particularly in processes which are relevant to safety.
The object of the invention is to specify a method of transmitting a measurement signal between a measuring unit and a control unit, in which the control unit can be used to recognize a malfunction in the measuring unit.
This object is achieved by a method of transmitting a measurement signal between a measuring unit and a control unit, in which a uniform measurement signal has a supplementary signal superimposed on it which alternates over time and is evaluated in the control unit to monitor the measuring unit, and, if the supplementary signal is erroneous, an alarm signal is produced.
This method ensures that a malfunction in the measuring unit is easily recognized and process control based on an erroneous measurement signal is prevented. This significantly increases the reliability of the process.
Advantageous further developments of the invention are specified in the subclaims.